The Fire Brigades Union (FBU) says the changes are unworkable and would mean firefighters working into their 60s, with the risk that they will face dismissal if they fail rigorous fitness tests.

General secretary Matt Wrack said: “It is absolutely ludicrous that a government can impose a scheme which means that firefighters will have to ride engines and rescue people from fires up to age 60.

“This is unsafe and unrealistic for both firefighters and the public.”

Fire minister Penny Mordaunt said the FBU action was “completely futile and counterproductive”.

She said: “My respect for firefighters and what they do, the safety of the public and the future of the service will be my motivation in office. This will not be altered or diminished by strikes or the threat of strikes.

“The action by the FBU is completely futile and counterproductive to the interests of their members and the public. I hope they will soon realise that and re-engage so we can get this situation swiftly resolved.

“I want to thank those unions who have continued to engage on these important issues and the huge efforts made by those firefighters, officers and staff in manning contingency arrangements during strike periods.” Firefighters will down tools between 12pm-2pm and 22.59pm-23.59pm every day from August 9-16.